As the demand for portable electronic devices such as personal digital assistants (PDAs), mobile phones, and notebook computers increases, so does the demand for portable devices having a more compact, thinner, and lightweight design. Accordingly, batteries providing power to those devices are regarded as an important component of such devices. In particular, lithium batteries have been used as major power sources for such portable devices because they are lightweight and have higher energy density.
Lithium batteries are comprised of, inter alia, cathode active materials and anode active materials. For example, U.S. Pat. Nos. 5,837,015, 5,635,151, and 5,501,548 disclose cathode active materials and anode active materials that may be used for lithium batteries. Cathode active materials for lithium batteries may be composed of Li-containing transition metal oxides, such as LiCoO2 and LiNiO2, and chalcogen compounds, such as MoS2. Since these compounds have a layer-like crystalline structure, Li ions can be reversibly intercalated or deintercalated into the structure. Accordingly, these compounds have been widely utilized as cathode active materials for lithium batteries. If, however, an anode active material comprises metal lithium which intercalates and deintercalates lithium ions, needle-shaped lithium dendrites grow on the surface of lithium. This occurs because the lithium repeatedly dissolves and precipitates during charging/discharging of the battery. As a result, the needle-shaped dendrites decrease charge/discharge efficiency, and cause internal short-circuits by contacting a cathode.
In order to overcome these problems, a material which reversibly intercalates and deintercalates lithium ions may be used as the anode material. This material may be lithium alloy, metal powder, graphitic or carbonaceous materials, metal oxides, or metal sulfides. When the sheet-type anode composed of a lithium alloy is used in a battery, however, the sheet-type alloy becomes thinner during charging/discharging As a consequence, the charge/discharge cycle characteristics of the battery deteriorate.
A binder is required when the sheet-type electrode is composed of metal powder, a carbonaceous material powder, a metal oxide powder, or metal sulfide powder, because these materials in powder form cannot form electrodes alone. For an example, Japanese Patent Laid-open Publication No. JP 4-255760 discloses a method of forming an anode using a carbonaceous material powder as binder, such as an elastic rubber-based polymer binder. A conductor is used in addition to the binder when the anode is composed of a metal oxide powder or a metal sulfide powder in order to improve the charge/discharge characteristics of the battery.
Additionally, lithium battery anodes also have been manufactured using an organic solvent containing N-methyl-2-pyrrolidone (NMP), but this compound is extremely toxic to humans and the environment. As a result, the manufacturing process is complex and requires more equipment in many processes. In order to overcome these problems, Japanese Patent Laid-open Publication No. JP 5-74461 discloses a method of forming an aqueous anode active material slurry using a styrene butadiene rubber (SBR)-based binder and a carboxymethyl cellulose-based binder. In this case, water is used as the solvent.
The performance of a lithium battery anode may be enhanced by improving the characteristics of the carboxymethyl cellulose-based binder. Such an improvement may be accomplished by adjusting the degree of etherification or polymerization of the carboxymethyl cellulose-based binder. The average degree of polymerization determines the viscosity characteristics of slurry. The degree of etherification indicates the number of hydroxyl groups which are capable of being substituted with a carboxymethyl group, but are actually substituted with carboxymethyl groups. For example, Japanese Patent Laid-open Publication No. JP11-67123 discloses a method of forming an aqueous anode active material slurry using a carboxymethyl cellulose-based binder which has a degree of etherification in a range of 0.5-1 and an average degree of polymerization in a range of 300-1,800. Moreover, Japanese Patent Laid-open Publication No. 2002-33105 discloses a method of forming an aqueous anode active material slurry using a carboxymethyl cellulose-based binder which has an average degree of polymerization in a range of 1,500 to 3,000 and where the product of the average degree of polymerization and the degree of etherification is in a range of 750-2,000.
Alternatively, the lithium battery anode may be improved by using a combination of a carboxymethyl cellulose-based binder and a SBR-based material to form the anode active material slurry. In general, a carboxymethyl-cellulose aqueous solution is prepared by dissolving the water-soluble carboxymethyl cellulose-based binder, subsequently the SBR-based material and an anode active material are added to the carboxymethyl cellulose aqueous solution and mixed to form the slurry. In this process, however, needle-shaped materials that are non-soluble in water are generated. The needle-shaped materials reside in the anode active material slurry which can amalgamate the electrode materials or can weaken the dispersibility of the slurry. Consequently, the adhesive force of the anode becomes weak, resulting in a lithium battery which exhibits a poor cycle life performance.
While much research on a conventional carboxymethyl cellulose-based binder has been focused on the average degree of polymerization and the average of etherification, little efforts have been directed to increasing the water solubility of the carboxymethyl cellulose-based binder to improve the dispersibility of the slurry and adhesive force of an anode, in order to attain a high performance lithium battery.